Jan Achenbach

Jan D. Achenbach is the recipient of the 2016 Sigma Xi William Procter Prize for Scientific Achievement.

He was born in the Netherlands, studied aeronautical engineering at Delft University of Technology, and received a PhD in Aeronautics and Astronautics from Stanford University in 1962.

He has made groundbreaking contributions to research on waves and vibrations in solid propellants, dynamic behavior of composite materials, dynamic effects on fracture, and applied ultrasonic methods for the measurement of elastic properties of thin films by acoustic microscopy, as well as for the detection of fatigue cracks and corrosion in aircraft, and recently for probabilistic methods of structural health monitoring.

With regard to practical applications, Achenbach was the leader of a team that developed an effective ultrasonic technique that dramatically reduced the inspection time for the detection of stress-corrosion cracks in the wing box of the DC-9. He was awarded the 1997 Model of Excellence Award by McDonnell-Douglas Aerospace.

He is the author of Wave Motion in Elastic Solids (Elsevier Science, 1973), and Reciprocity in Elastodynamics (Cambridge University Press, 2003), as well as three other books and numerous papers in technical journals. Some 60 PhD dissertations have been completed under his supervision. In 1993, he was elected to the Chicago Tribune All-Professor Team for his teaching and mentoring.

In 2014, he received the Monie A. Ferst Award, a national award from the Georgia Institute of Technology Sigma Xi Chapter that honors science and engineering teachers who have inspired their students to significant research achievements.

He is a member of the National Academy of Engineering, the National Academy of Sciences, and a Fellow of the American Academy of Arts and Sciences. He was awarded the 2003 National Medal of Technology and the 2005 National Medal of Science.

Interview with Jan Achenbach

Transcript of Interview

Heather Thorstensen: Hello everybody, and welcome to this interview from Sigma Xi, the Scientific Research Society. My name is Heather Thorstensen, and I am the manager of communications for Sigma Xi. Today I'm speaking with Professor Jan Achenbach. He is the 2016 recipient of Sigma Xi's William Procter Prize for Scientific Achievement. He is also the Walter P. Murphy and Distinguished McCormick School Professor Emeritus of Civil and Environmental Engineering, Engineering Sciences and Applied Mathematics and Mechanical Engineering at Northwestern University in Evanston, Illinois.

The Procter Prize for Scientific Achievement is a prize that Sigma Xi gives out to recognize a scientist who has made an outstanding contribution to scientific research, and who has also demonstrated an ability to communicate that research to scientists in other disciplines. Thank you for speaking with me today, Professor Achenbach. Let's start by talking about how you got into your research area which is about mechanical waves. How did you get started in that area?

Jan Achenbach: I was a graduate student at Stanford University, and my project was waves and vibrations. This was just shortly after Sputnik, and the United States was making a big attempt to catch up with the Russians and put their own object in space. I was in the Department of Aeronautics and Astronautics at Stanford University, and I was asked to study waves and vibrations in solid components. I did that, and that was the topic of my PHD dissertation, and then when I was finished my interest in waves and wave propogation in solids continued over many many years. I'm still interested in waves in solids.

Thorstensen: Your research in waves in solids led to two national medals. You got the National Medal of Technology in 2003 and the National Medal of Science in 2005. Could you talk about the research about wave propagation in solids that led to those medals?

Achenbach: Yes that is a field which is called non-destructive evaluation, and the purpose of that field was to detect flaws in structures, any structures. Could be airplanes, bridges and nuclear reactors. We use the terminology safe, critical structures. If one of those structures would fail, possibly a good many people would die in the failure.

It is a field that makes sure that planes are safe, at least safe from structural failure. That bridges are safe and that nuclear reactors are safe. That is called non-destructive evaluation. I became interested in that field back in the late '70s. At that time, the field was of a more qualitative nature. As you probably know, ultrasound is a high-frequency sound that can penetrate solids, and that can, as it penetrates the solids will be a reflected or scattered by flaws. That is the purpose of non-destructive evaluation, to detect flaws and other undesirable and the flaws in structures.

I did that for aircraft in the beginning, and we did that for a number of years. What we did was in the first place, we made the field more quantitative. When I started off, the field was qualitative. The equipment was available, and there were very competent technicians who would look at signals that were generated by that equipment. They could tell that there was something, but they couldn't tell the details of it. They couldn't tell whether it was a crack, or a small cavity, where it was and how big it was.

The field had to become quantitative. The Q had to be added, and it had to become QNDE. I worked on that at the very beginning, and developed ways of doing that and setting up theoretical work that would make it possible for ultrasound to be very specific and to characterize solids. I might mention that after I finished at Stanford and before I did this, I wrote a book. This was called Wave Propagation in Elastic Solids. It was published in 1973. It's still available in paperback, and some people call it the Bible of waves in solids. The work that I did for quantitative non-destructive evaluation was very much based on the discussions that I have presented in my book.

Once you had done a lot of theory, we moved over to verification and to do new tests. We did a number of tests, but I'll give you an example of the sort of tests that turned out to be very useful for the airline companies.

At that time, which was back in 1995, there was a plane which was called the DC-9, and it was made by McDonnell Douglas Aerospace. The company does not exist anymore. It was taken over by Boeing. They flew that plane, and then they wanted to test the wing for stress corrosion cracking. To do that, they had to take off the complete wing and send somebody with a basically a flashlight to find out if there was stress corrosion cracking. What we did was we developed an ultrasonic method that did not require to take the wing off but to do it from the outside of the wing and to detect cracks and corrosion that way. That was very useful for McDonnell Douglas. That kind of activity eventually led to me receiving the medal, the National Medal for Technology. Whereas the theory that I had developed actually earlier led to the National Medal of Science which I got 2 years later.

Thorstensen: It's all about making sure that you can understand how many flaws are in there as well as being able to do this in such a way that you don't have to take apart the structure in order to find out how many there are?

Achenbach: That's right. You should still go a little further. If you can find out how many flaws there are, you should also be able to make a statement whether a repair is required. Whether a structure for a plane should be taken in for maintenance and for a bridge should be repaired just on the bridge. The same for nuclear reactor. That has become an important part of the field also, and it requires a lot of interaction with material science and your engineers from material science departments. The field has become more interdisciplinary over the years in addition to material scientists, we also need electrical engineers, because the sensors have become very sophisticated and the process and techniques have become very sophisticated. We cooperate with electrical engineers to get the best results.

Thorstensen: Could you explain what it is about the ultrasound and how it actually works to find these flaws?

Achenbach: Ultrasound is sound, and sound propagates as you know. It takes sound to go from one source to a receiver elsewhere, even when people speak to each other. Sound is defined by frequency. There is low-frequency sound, and there is high-frequency sound. Low-frequency sound is the sound that we ordinarily use in talking to each other. High-frequency sound is sound with a frequency higher than 20,000 cycles per second. Frequencies are measured in cycles per second. They are sometimes called Hertz after a scientist in the late 19th century.

Ultrasound is sound higher than 20,000 cycles, and whereas most sound is reflected by solid surfaces, ultrasound penetrates solid surfaces. If you think of a comparison with glass, light penetrates glass, but it can also be reflected by glass. Ultrasound penetrates a solid material like a metal or the body of an individual to detect tumors, to make images. Ultrasound can do that, because it has this high frequency and the correspondent short wave length. It makes it possible to have it penetrate solid bodies. At that high frequency, it can only be generated by electronic equipment or by lasers, but not by any other ways.

Thorstensen: Is it always ultrasound frequencies that you're using when you're doing the quantitative non-destructive evaluations?

Achenbach: Yes, sometimes at very high frequencies in the megahertz sense. Megahertz is a million cycles per second. That's very high for certain materials to have to use that. For other materials we use 50,000 cycles or 200,000 cycles. There's a whole spectrum of frequencies that is being used for different structures.

Thorstensen: Okay and how have you been able to communicate the significance of your research on wave propagation in solids to other disciplines?

Achenbach: As I told you, I wrote a book which got a lot of circulation and was used in courses. I attended meetings every year even when I was much younger than I now am. Overall there was a good many meetings in different areas. That was one way of doing it. I am at the university, and we have departments of material science and engineering and a department of electrical engineering. It is very easy for me to get in touch with persons, professors in other departments and to discuss with them certain problems that I might have.

Thorstensen: What sort of other scientific disciplines has this research moved into? I read about structural health monitoring and other uses besides aircraft or building structures.

Achenbach: Let me explain the difference in what is generally called non-destructive testing and structural health monitoring. For non-destructive testing, let's say of a bridge, the equipment is taken and the sensors attached to the bridge as the equipment is there. The same for most of the time for nuclear reactors. A plane, you would take into a maintenance facility and use equipment in the maintenance facility. That was the original way, and it's still being done most of the time that way. Now that more sophisticated and smaller sensors are becoming available, these sensors are permanently installed on aircraft, or on bridges or on nuclear reactors. The information can be wirelessly transmitted to a central receiving point and then to a point where it can be processed. Already, to some extent, in the future a lot more it will not be necessary to go out to bridges and nuclear reactors and to bring planes in, because a lot of information can be obtained from the sensors that are permanently installed in those structures.

Thorstensen: When you're talking about structural health monitoring, those are the sensors that are on their all the time? That's what that means?

Achenbach: That's right.

Thorstensen: I also read about non-destructive testing and evaluation in quality engineering. What is that part of it?
Achenbach: Let's make sure that products that companies sell don't have flaws in it and are properly done. Before, let's say, an automobile company would sell a car they might check a few locations to make sure that everything is in good shape. That's quality control which is done by the use of non-destructive testing.

Thorstensen: One thing that we didn't talk about was the big application that you had for your wave propagation in solids research with the Aloha flight [an Aloha Airlines flight in 1988 in which the roof tore off the plane while in flight, killing one flight attendant and seriously injuring eight people]. Let's make sure that we talk about that. What was it that brought you into that situation?

Achenbach: I learned about it, and I was immediately interested in it. I had studied aeronautical engineering, but I was interested in planes. Then more information came available, and then the FAA became interested. The FAA's the Federal Aviation Administration. They're responsible for the safety of aircraft. They have regulations and rules of how planes have to be maintained, and what you can do with a plane and what you cannot do with it.

The Federal Aviation Administration immediately started a number of projects to really find out what had happened. I applied for one of those and was funded by the FAA. I started work in that area, and with other people. I was not the only one. We figured out what had happened and how it could be prevented. I can tell you in detail how it happened if you like, but the main thing is that Boeing who had manufactured that plane made appropriate changes in the manufacture of the plane so they didn't have it happen again.

That was actually a 737, a B737 which in due time became the most popular plane that Boeing ever made, and they sold more of them than any other plane. Everything worked out after the particular failure had been recognized and had been corrected. I can add one thing to my discussion of this, which I did years later. When you do this work I did in non-destructive testing, it's sometimes very difficult to work with specific numbers. Everything is not really what we call deterministic. You definitely do not know everything when you set up a test procedure.

You do two things that I didn't talk about. You design a measurement model or a specific testing procedure. That measurement model includes everything. It includes the equipment to generate the ultrasound. It includes ways the ultrasound enters its structure. It includes the ways how it's being and how it interacts with us and it includes the ways it is measured. We call that a measurement model. I was very active in developing measurement models. That probably played a role in the National Medal of Science.

Later on I realize that nothing was specific and we can use probabilistic techniques, in other words a certain property might not be known exactly precisely with a specific number. It might be known over a small range of possibilities, and if you do that then you enter the range of probabilistic analysis. You don't say anymore that something either fails or not fails, but you say there is a certain probability. Hopefully a very small probability that something had failed. If it's not small enough, then you have to do something. That's the work I have done recently, not recently but over the last 3 years on the Aloha problem.

Thorstensen: Was the reason that the top of the aircraft came off mostly because of cracks in the aircraft?
Achenbach: Yes. In aircraft you have lap joints. Can you see me? This is a lap joint [he places one hand on top of the other]. One plate another plate and there is another region in between here the connection is made. The original plane had made that connection with an adhesive, and they they had reinforced it with rivets. In Hawaii there's a lot of humidity in the air, and there's also a lot of sodium and salt in the air. The salt got in that lap joint and deteriorated it.

Then the loads had to be taken over by the rivets, and these rivets had countersunk rivets. That's a little bit more difficult to explain. I think a lot of people who listen to me will know what countersunk rivets are. They produce so-called stress concentrations. If you have concentrations, and if you have a plane that takes off frequently as it does when you fly from one little island to the next one in Hawaii, then you have lots of cycles on such a plane.

There is a phenomenon called fatigue whereby metals can fail without really being subjected to very high loads, just the fact that you have changing loads. If this were flexible, and if I would be able to bend it and if I would bend it many, many, many times. You can try it with a coat hanger and you will find out it will break eventually even though you're not pulling all that hard.

That's the phenomenon of fatigue, and the fatigue created little cracks coming out of a rivet hole. There are neighboring rivet holes and then the cracks coming out of two neighboring rivet holes propagates towards each other and creates a tear. Once you had one tear you soon got a bigger tear. The tear went all the way by the first class until it reached a very strong area near the wing. Then [it went towards] the other side, and it ran towards the cockpit and the area near the top of the cockpit cabin between the cockpit and the wing blew off because of the tear on both sides. This was a fantastic pilot. It happened at 20,000 feet, and only one person died, the cabin attendant who was standing in the aisle. She was sucked out of the plane, but the pilot was able to land the plane. It was possible to find out exactly what had happened.

Thorstensen: Was it solely because of the quantitative non-destructive evaluation technique that you developed? Is that the reason that they found out what happened to this plane or were there other projects going on at the same time that helped?

Achenbach: They found out what happened to the plane when the plane had landed. The non-destructive testing technique was designed to prevent it ever happening again.

Thorstensen: I saw, also, one other thing that you did in your career was you started a journal called Wave Motion. I was wondering what kind of contributions have you seen come out of that journal to the field of the wave propagation in solids?

Achenbach: Well I had the idea in 1978, and I knew that people in many fields were writing papers about wave motion to apply to their specific problems. The foundation was often the same. They used the same basic equations. They just had different applications, and there were also many papers in fluids that involved waves. I thought it would be nice if there were a journal for people who did anything in waves whether it was a non-destructive evaluation in geophysics and other fields, medical fields with the use of waves to image. We'd have a journal where they could publish their result and to make the results known to persons who were in other areas. This Wave Motion journal was supposed to deal with basic ideas in wave propagation applied to different areas. It still is. I'm no longer the editor in chief, but it still has the same idea.

Thorstensen: Did you see any interdisciplinary science come out of that? Were people actually making connections because they were reading different papers that they normally might not have seen about waves, and they were applying it to what they were doing?

Achenbach: Yes, I think so. I think I personally learned a lot about papers that were in the area of earthquake engineering of geophysics that were published in the journal. After a short time, I appointed an individual in the field to be a member of the board of editors so we would get more papers in that area in the journal. That worked out very well, and I did the same thing for other areas. I made a board of editors that had persons from very different fields in it, and they would deal with papers in their own field, and then have them reviewed and pass them onto the journal.

Thorstensen: That's a smart way to get the papers that you need to fill in what you're missing.

Achenbach: That's right.

Thorstensen: Could you talk about the value that you see in getting that interdisciplinary science together?

Achenbach: It makes everything that is being done more efficient and more effective. It also is a good way to get things going sooner and earlier. In fact, at Northwestern University we try to bring in students from different departments in certain courses so they can get to know each other and start working together at the very early stage. Science and engineering and science and technology is becoming more and more complicated. It is really necessary in many cases to use an interdisciplinary approach.
Thorstensen: I also read about another area of your research. I'm not sure if it's very familiar with what we already talked about, but it's that you developed methods for thin layer characterization by acoustic microscopy.

Achenbach: Yes. You're familiar with it, many people are familiar with an acoustic microscope which enlarges what you see, what can be seen at the surface of a solid. An acoustic microscope penetrates the solid, but still acts as a acoustic microscope. It's main difference with other techniques is that it focuses. If you have a curved lens, like an optical microscope has, you can focus light. If you have a curved transducer, a transducer is a device that produces ultrasonic sound, you can focus off the sound. You can focus off the sound under the surface of a solid, and we use that determine the mechanical properties of thin layers to get information there of inside those layers of material. We have an acoustic microscope here at Northwestern.

Thorstensen: Could you explain what you mean by a thin layer?

Achenbach: Thin layers can be used for various reasons. Thin layers can be used to protect the surface of structures against thermal effects if there is a lot of heat. Thin layers can also be used in electronic devices that actually play an important role in producing the output of an electronic device. They have multiple applications.

Thorstensen: I also saw a big thing that your nominators wrote about you was how much you do for mentoring students. Mentoring is a big thing at Sigma Xi where we try to promote mentorship, and I wondered if you had any advice for other mentors out there?

Achenbach: It's true. I've done quite a bit of mentoring. I've done mentoring of graduate students, of post-docs and now and then of young faculty members. At universities, faculty members have projects which are funded by agencies of the federal government, by the state or by companies. Graduate students work on those projects as research assistants, and that's how they can afford to go to graduate school. They don't have to pay. They can just work for the higher degree, like the master's degree or the PHD degree, but they do have to do research on a project.

That's a limitation that limits mentoring a little bit, because you have to tell students at the very beginning that you will respect their individual ideas and you want them to have individual ideas, but they have to have them within the context of these projects. I've always given my students a lot of freedom and a lot of leeway to do what they like. The only thing I really have to insist was that what they did would fit into my projects. The same thing for post docs. That has worked well for me.

Other things I do is I make sure that graduate students learn how to write. I ask them to write papers that we coauthor at least first drafts. I ask them sometimes even to write a contribution to a proposal, and they learn to write before they ever leave here. I also ask them to make presentations. To give lectures for my group that work with me or at meetings, national and international meetings. I think that's very important also for students before they get their degree.

Finally, if there was a student who wanted to become a faculty member himself after he was finished here, I would try to arrange that he would do some teaching here. All that kind of can add up, and I think those are good things to do. You should never put too much pressure on students or to bully them. That's one thing you certainly should never do, and I know there have been faculty members here and there and the other place where that has happened. It's a very bad thing to do.

The faculty members, they also can use a little mentoring when they start off, because they don't have all that much experience in teaching. If they start teaching a course that I have taught myself in the past, I can tell them the difficulties are that students usually have. I can tell them what to do and what not to do. One important thing that I also often tell them is that they should not overestimate how smart students are. If students are smarter than you think they are, you find out pretty soon and you can adjust. If they are not, you can get [behind] because they can't keep up, and you have to go slow down, and you have to do all kinds of make up things and so forth. Make sure that you properly understand the level of knowledge of your students.

Thorstensen: We also have Sigma Xi members who are students, so they're probably wondering how to find a good mentor and how to approach a mentor. What are some of the ways that you have appreciated being approached when you were asked to be a mentor for a student?

Achenbach: That's another good thing we do at Northwestern. Students come in and at the very first weeks, we organize presentations by all the faculty members that are able financially to support students or that are otherwise invested in having students work with them. I'm talking about graduate students, and so students get to know a little bit about these professors just by listening to them. After that, still in the first quarter, they can visit a number of students in their offices, and they can discuss the kind of research that they would do. The general area of the research that it would be if they would work with that specific professor. For them the graduates have time, they have the whole year and that would [be enough time for] them to make up their mind what they're going to do.

Thorstensen: It sounds like you would want a student to come to you and really learn about your research before deciding to work with you as a mentee.

Achenbach: That works both ways. I also want to know something about the students before I say, okay work with me.